The present invention relates to a method of producing a plate-shaped body having a flat surface of a high precision from a thermoplastic resin. More particularly, the invention is concerned with a method suitable for use in the production of video discs which are required to have minimized circumferential warp in the thicknesswise direction, high uniformity of thickness and small distortion of the signal track.
Typical conventional process for producing video discs comprises the steps of preparing a video-disc pattern (referred to as "stamper", hereinafter) in which information is stored in the form of a spiral groove, pressing the stamper against a video-disc material which is usually a thermoplastic resin such as polyvinyl chloride, while heating the video-disc material to a termperature above the softening point thereof, and decreasing the pressure, after cooling, to thereby obtain a video disc to which the information has been transcribed from the surface of the stamper.
This conventional process, however, is not satisfactory because it leaves a considerably large circumferential warp and uneven thickness distribution.
The present inventors have learned through experiments, that a high-pressure press at a temperature below the Vicat softening point causes an uneven pressure distribution which in turn creates a residual stress in the formed disc resulting in a large distortion of the disc. It was confirmed that, non-uniform high pressure at a temperature below the Vicat softening point causes a residual stress due to contraction of the video disc to permit warping of the disc.